1. Field of the Invention
This invention relates to a poly(vinyl chloride)-based (hereinafter referred to as "PVC-based" for the sake of brevity) resin composition, and more specifically to a PVC-based resin composition having highly-balanced properties between impact resistance, transparency and stress-whitening resistance and excellent processing characteristics.
2. Background of the Invention
PVC-based resins have a drawback that they are brittle to impact. As methods for improving this drawback, many proposals have heretofore been made that various graft copolymers obtained by graft-polymerizing one or more vinyl monomers such as styrene, methyl methacrylate and acrylonitrile in the presence of a rubbery butadiene-based polymer are blended in PVC-based resins. PVC-based resins according to these methods have been improved certainly in impact resistance, but still involve such a drawback that the inherent excellent transparency of the PVC-based resins has been impaired or when molded articles are bent or an impact is exerted on molded articles, portions subjected to such a stress are whitened (stress whitening) and their commercial values are hence impaired. Especially, the above drawback becomes more remarkable as the impact resistance become better. These properties can be attributed to the fact that the impact resistance of a PVC-based resin composition is dependent basically on the particle size of rubber particles in a rubbery polymer component which forms a graft copolymer. In order to enhance the impact resistance imparting ability of a graft copolymer, it is generally necessary to increase the particle size of a rubbery polymer to a suitable extent. In this case, the transparency tends to be impaired due to the increase of degree of scattering at interfaces between a PVC-based resin and particles of the graft copolymer dispersed in PVC-based resin. Even if the refractive index of the PVC-based resin and that of the graft copolymer are brought into confirmity as much as possible, the stress whitening is also liable to occur.
With a view toward obtaining a PVC-based resin composition excellent in impact resistance, transparency and stress-whitening resistance, a variety of attempts have hence been made on the composition of a rubbery polymer component, its particle size, the composition of components to be graft-polymerized, the method of graft polymerization, etc. as parameters of a graft copolymer to be blended.
As illustrative examples of the graft copolymer to be blended in the PVC-based resin, may be mentioned a graft copolymer obtained by coagulating a rubber latex to an appropriate extent and then graft-polymerizing styrene, methyl methacrylate or the like (Japanese Patent Publication No. 31462/1971 or 1584/1979); a graft polymer obtained by graft-polymerizing a monomer mixture of an alkyl methacrylate and an aromatic vinyl compound in the presence of a latex containing highly crosslinked rubbery butadiene-based polymer having a degree of swelling of not greater than 7 (Japanese Patent Laid-Open No. 212246/1982); a graft copolymer obtained by using an elastomer, which contains 1,3-butadiene, n-butyl acrylate and styrene at a specific content ratio, as a rubbery polymer component and then polymerizing one or more graft-copolymerization components in multi stages (Japanese Patent Publication No. 23648/1972 or 23649/1972 or Japanese Patent Laid-Open No. 150562/1976); a graft copolymer obtained by graft-polymerizing styrene or methyl methacrylate on a backbone polymer composed of butadiene, acrylonitrile, styrene and a crosslinkable monomer (Japanese Patent Publication No. 5787/1973); and a graft polymer obtained by graft-polymerizing an aromatic vinyl compound, methyl methacrylate, acrylonitrile or an acrylic acid ester on a rubbery polymer (Japanese Patent Publication No. 42021/1971).
However, these attempts cannot still be said to be satisfactory. It has hence been desired to develop a PVC-based resin composition which is still highly balanced between properties of impact resistance, transparency and stress-whitening resistance and is also superb in processing characteristics.
In addition, PVC-based resin compositions containing these graft copolymers are accompanied by processing drawbacks such that their impact resistance is reduced when formed or molded at higher temperatures or fish eyes of an ungelled matter are formed to impair the external appearance when formed or molded under conditions of a low kneading degree. There is hence an outstanding demand for a PVC-based resin composition which can exhibit high impact strength stably and has excellent external appearance over a wide range of forming or molding conditions.